1. Field of the Invention
The present invention relates to an element substrate for an inkjet printhead, a printhead using the element substrate, and a head cartridge having the printhead.
2. Description of the Related Art
The electrothermal transducers (heaters) of a printhead mounted in an inkjet printing apparatus, and their driving circuit and wiring pattern are generally formed on a single substrate using a semiconductor process technique. A known example of the printhead having this arrangement is one disclosed in U.S. Pat. No. 7,216,960.
FIGS. 18 and 19 are schematic views showing an example of a conventional element substrate for an inkjet printhead.
FIGS. 18 and 19 characteristically show the same part of the element substrate. FIG. 18 mainly shows a heater driving power supply wiring pattern and ground wiring pattern. FIG. 19 mainly shows a heater driver, logic wiring pattern, and logic circuit below these wiring patterns.
The arrangement of respective components will be explained first with reference to FIG. 19. An ink supply port 604 is formed at the center of the element substrate, and heater arrays 807 are arranged on the two sides of the ink supply port 604. Ink channels, and orifices for discharging ink are formed in the element substrate in correspondence with respective heaters, and ink is supplied to them via the ink supply port 604. A driver array 901 is arranged on the outer side of each heater array 807. A logic circuit wiring pattern and logic circuit 106 are arranged on the outer side of each driver array 901. Connection terminals 905 are arranged near each short side of the element substrate. A shift register (S/R) 903, a decoder 904, a temperature sensor (not shown), and the like are interposed between the connection terminals 905 and the ink supply port 604.
In FIG. 18, a heater driving power supply wiring pattern 803 is arranged above each driver array 901. A ground wiring pattern 804 is arranged above each logic circuit 106. These wiring patterns are connected to the outside via heater driving connection terminals 801 and ground connection terminals 802, respectively.
Heaters in the heater array are driven by a so-called time division driving method of shifting the driving timing for each block of simultaneously drivable heaters.
In order to make the wiring resistances of arrayed heaters almost equal to each other, the power supply wiring pattern is divided for each driving group of heaters not driven simultaneously. The respective wiring patterns have different widths in accordance with the distance from the connection terminal, so as to make resistance values almost equal to each other. For example, a wiring pattern having a longer distance and larger wiring length has a larger width. In each driving group, the number of simultaneously driven heaters is one, so the voltage drop by the wiring resistance is almost equal between heaters.
In FIG. 19, the connection terminals 905 are arranged near the two short sides of the element substrate. This is because the wiring width becomes excessively large if the connection terminals 905 are arranged on only one short side of the element substrate and the wiring pattern extends up to the other short side. As shown in FIG. 18, the power supply wiring patterns are symmetrical in the longitudinal direction on the sheet surface of FIG. 18. That is, the heater driving connection terminals 801 and ground connection terminals 802 are necessary on the two short sides.
Terminals other than the heater driving connection terminal 801 and ground connection terminal 802 are used as a heater driving enable terminal, data input terminal, latch terminal, clock terminal, logic power supply terminal, temperature sensor terminal, rank measurement terminal, and the like.
These days, inkjet printing apparatuses are demanded for higher printing resolutions and higher printing speeds. The element substrate for an inkjet printhead needs to be elongated to cope with a higher-density arrangement of heaters and logic circuits, a larger number of orifice arrays corresponding to a larger number of ink colors, and a larger number of heaters. As a result, the area of the element substrate increases, raising the cost.
FIG. 5 is a plan view of an example of the element substrate of an inkjet printhead. As represented by an orifice portion 501 in FIG. 5, a plurality of types of orifices having different orifice diameters and the like are arranged so that ink of the same color can be discharged by different discharge amounts. Known examples of the printhead having this arrangement are ones disclosed in U.S. Pat. No. 6,137,502 and Japanese Patent Laid-Open No. 2007-144711 (WO2007/061138).
Referring to FIG. 6 which is an enlarged view of the orifice portion 501, orifices 602 of array A have a discharge amount of 2 pl and an array density of 600 dpi. Orifices 601 of array B have a discharge amount of 2 pl and an array density of 600 dpi. Orifices 603 of array C have a discharge amount of 1 pl and an array density of 600 dpi. Arrays B and C are positioned on the same side of an ink supply port 604, and orifices are staggered. The orifices of arrays B and C are arrayed at an array density of 1,200 dpi, which is substantially double that of array A. In other words, orifices are formed at an array density of 600 dpi on one side of the ink supply port 604, and those are formed at an array density of 1,200 display on the other side.
FIG. 10 is a view schematically showing the element substrate of the orifice portion 501 in FIG. 5. As shown in FIG. 10, heaters 104 corresponding to the orifices 602 of array A are arranged on one side of the ink supply port 604, whereas heaters 103 corresponding to the orifices 601 of array B and heaters 105 corresponding to the orifices 603 of array C are arranged on the other side. A driver 101 corresponds to each heater 103, a driver 102 corresponds to each heater 104, and a driver 107 corresponds to each heater 105. Reference numeral 106 denotes each logic circuit. In the array of the drivers 102, the drivers are arrayed at an array density of 600 dpi. In the array of the drivers 101 and 107, the drivers are arrayed at an array density of 1,200 dpi.
Problems will be described, which arise when a plurality of orifice arrays having the same discharge amount exist on a single element substrate, and the drivers of respective driver arrays are formed at different array densities in the respective driver arrays corresponding to the respective orifice arrays. The following description assumes that the driver is a transistor.
FIG. 11 shows the arrangement of the heater driving power supply wiring patterns 803 and ground wiring patterns 804 which are superposed on the circuit shown in FIG. 10 via an insulating film.
The heaters 103 and 104 discharge ink droplets in the same discharge amount of 2 pl. To make discharge characteristics such as the discharge amount and discharge speed equal to each other, driving conditions are desirably made equal. That is, the heaters 103 and 104 are desirably driven with the same pulse using the same heat enable signal which defines the period during which the heater is driven.
The number of heat enable signal terminals is desirably small in order to downsize the element substrate. A small number of heat enable signals is advantageous even in cost because the printing apparatus main body need not have many pulse tables.
To make discharge characteristics such as the discharge amount and discharge speed equal to each other, and share the heat enable signal, it is desirable to make the size equal between heaters and make the ON resistance and wiring resistance equal between drivers. In FIG. 10, the drivers 101 and 102 have the same width in the driver array direction and the same length L1 in a direction perpendicular the driver array direction, and have the same size. Thus, the drivers 101 and 102 have the same ON resistance, and the heater driving power supply wiring pattern 803 and ground wiring pattern 804 in FIG. 11 have the same sizes and wiring resistances for both the heaters 103 and 104. In this case, the heaters 103 and 104 can be driven by the same heat enable signal, and attain the same discharge characteristics.
However, as is apparent from the drivers 102 in FIG. 10, they are arranged in accordance with a 1,200-dpi heater array though they are originally arrayed at 600 dpi. A gap is generated between adjacent drivers, resulting in poor arrangement efficiency, that is, an unnecessarily large chip size.
FIGS. 12 and 13 are schematic views, similar to FIGS. 10 and 11, and show an example of improving the driver arrangement efficiency.
The drivers 102 are arrayed at 600 dpi, similar to FIG. 10. In order to make the ON resistance of the drivers 102 equal to that of the drivers 101 arrayed at 1,200 dpi, the length of the driver 102 in a direction perpendicular to the heater array direction is halved while the area of the driver 102 is kept constant, preventing generation of an unnecessary space.
In this case, as shown in FIG. 13, each driver needs to be connected on the outer side to the ground wiring pattern 804. The wiring width of the heater driving power supply wiring pattern 803 is narrowed in accordance with the driver 102. Although the area above the drivers 101 and 107 is sufficiently large, the wiring width is narrowed to set the wiring resistance of the driver 101 equal to that of the driver 102 and make discharge characteristics equal to each other.
In this case, the driver arrangement efficiency can be increased to downsize the element substrate, but the wiring resistance rises, decreasing the electrical efficiency.
As described above, when a plurality of orifice arrays having the same discharge amount exist on a single element substrate, and transistors which form respective driver arrays are formed at different array densities, it is difficult to achieve both a small-size element substrate and high electrical efficiency.